1. Field of the Invention
The present invention relates to a fixing device used in an image forming apparatus, such as a copying machine, a printer, and a facsimile machine, and to an image forming apparatus equipped with the same.
2. Description of the Related Art
Generally, an electrophotographic image forming apparatus, such as a copying machine, is provided with a fixing device as means for applying heat and pressure onto a toner image formed on a sheet to fuse toner particles and fix them on the sheet. There is known a fixing device that includes a heating member in the shape of an endless belt heated by a heat source and a pressure member driven to rotate while being pressed against the heating member for driving the heating member to rotate. Both of the heating member and the pressure member function as nip members. Specifically, both of the members have portions which are pressed against each other to form a nip portion. When a sheet with a toner image transferred thereon enters into the nip portion, the heating member heats the toner image and the pressure member applies a pressure to the toner image, which fixes the toner image onto the sheet.
In the fixing device configured as above, the heating member is formed in a belt-like shape (including a film-like shape) and has a small thickness, which enables heating the heating member quickly to a toner fusible temperature. This shortens waiting time of the user after the turning-on of the power or resumption from the sleep mode of the image forming apparatus, and thereby enhances the convenience for the user. This also makes it possible to stop supplying power to the heat source when the image forming apparatus is not used or to set a temperature of the fixing device lower than before when it is not in use, thus making an advantage of saving power consumption of the image forming apparatus.
The heating member in the shape of an endless belt, however, possibly leans to one side from the center position in the axial direction orthogonal to the traveling direction of the belt or wobbles while it is running. Such leaning or wobbling of the heating member is attributed to various factors, and is chiefly caused by unevenness of the nip pressure in the axial line direction between the heating member and the pressure member, which is introduced by low parallelism between the axial line of the heating member and the axial line of the pressure member. Leaning to the axial direction or wobbling of the heating member causes wrinkles in a sheet, poor fixing, and jamming resulting from disturbed transportation of a sheet. Further, serious displacement or wobbling might break an end portion of the heating member.
In order to prevent such displacement or wobbling of the heating member (belt) in the fixing device, Patent Document 1 (JP-A-2005-157172) discloses a heating device including a belt-like rotating body, a guide member that guides the rotating body, a pressure member pressed against the rotating body, heating means for heating the rotating body, a flange that supports the rotating body on the both sides, and an end portion holder that supports the flange and has a portion on which a part of the flange strikes. The end portion holder determines the positions of the both ends of the rotating body via the flange, thereby preventing the rotating body from leaning to the axial direction (see Claim 1 and paragraph 0025 of Patent Document 1).
The flange seems to be effective for a certain degree of leaning or wobbling of the heating member. However, when a extremely large force biasing the heating member to the axial direction presses the heating member against the flange, the force may finally break the end portion of the heating member. In short, when a force pressing the heating member against the flange is greater than mechanical strength of the heating member, breakage of the heating member is not avoidable. Such breakage shortens the life of the fixing device.
An inconvenience as described above can also happen in another fixing device in which the heating member and the pressure member are reversed, that is, in a fixing device in which a second nip member in the shape of a roller pressed against a first nip member in the shape of an endless belt corresponds to the heating member heated by the heat source, and the first nip member corresponds to the pressure member.